The present invention relates generally to protection switching and to mechanisms for protecting end-to-end communications at the traffic layer. In particular, the invention is concerned with techniques for allowing fast initiation of protection switching within a protected domain of a communications network.
Asynchronous Transfer Mode (ATM) is increasingly becoming the traffic layer of choice for transmitting information among nodes in a network. Better known by its acronym, ATM is a cell-based switching and multiplexing technology designed to be a general-purpose, connection-oriented transfer mode for a wide range of services. The basic building block of an ATM network is the ATM cell.
Each ATM cell sent into the network contains addressing information that establishes a virtual connection from origination node to destination node. Different ATM virtual connections may share the same physical link but belong to different classes of traffic, such as constant bit rate (CBR), variable bit rate (VBR) or unspecified bit rate (UBR). As a result, ATM has the ability to accommodate data and voice traffic having varying bandwidth and quality of service requirements.
ATM traffic can travel between end points along one or more virtual channel connections (VCCs). Different VCCs may thus share the same origination and destination nodes but may be associated with their own quality of service and bandwidth requirements. Also, as is known in the ATM art, several VCCs can be associated with a single virtual path connection (VPC), which defines a unidirectional flow of ATM cells from one end user to another. Moreover, it is possible to define multiple VPCs between two end points, each of which can be associated with multiple corresponding VCCs.
In many situations, such as when high reliability is required, it becomes necessary to protect end-to-end ATM traffic in the event of a failure. Accordingly, the art has seen the development of switching mechanisms for protecting individual VPCs/VCCs. This may consist of providing a protected domain between the origination node and the destination node.
The protected domain typically consists of a bridge node for sending traffic along a working path and/or along a physically diverse protection path. At the other end of the protected domain is a selector node, for selecting either the traffic from the working path or the traffic from the protection path. The selected traffic is passed onwards to the destination node. Thus, although the working path may fail for a given VPC/VCC, the VPC/VCC need not be dropped entirely if the protection path is functional and available.
However, as the number of virtual connections (VPCs/VCCs) that can be accommodated on a single physical link increases, it becomes prohibitively complex to individually protect the hundreds and possibly thousands of connections which share the working path through the protected domain.
In recognition of this difficulty, a bundling concept has been introduced, whereby connections traversing the same physical route are protected as a group. For example, multiple VCCs and VPCs having the same source and end points within the protected domain can be associated with a single virtual path group/virtual circuit group (VPG/VCG). This simplifies the task of protecting a large number of connections.
One such protection scheme is described in a document entitled xe2x80x9cITU-T Recommendation I.630, ATM Protection Switchingxe2x80x9d, published by the International Telecommunications Union (ITU) Telecommunication Standardization Sector in March, 1999 and hereby incorporated by reference herein.
Specifically, recommendation I.630 discusses the use of an automatic protection switching (APS) channel in both the working and protection paths. The APS channel is a virtual connection for control purposes, defined over the extent of the protected domain and contained within a VPG/VCG. Its purpose is to assist in evaluating the quality of the VPG/VCG and to serve as a conduit for protection switching control protocol messages.
Using the method disclosed in recommendation I.630, a node detecting a failure inserts alarm messages into all the VPCs/VCCs of the group, including the APS channel. The selector node of the protection domain receives these alarm messages and initiates protection switching, while forwarding alarm messages to the destination node outside the protected domain. Of course, the end-to-end connection undergoes an interruption during the time when alarm messages are being received by the destination node.
In many cases, the number of VPCs/VCCs in the group and the distance separating the node detecting the failure from the selector node can be considerable. This results in a lengthy delay in the selector node receiving the alarm message over the APS channel, correspondingly resulting in a lengthy delay before protection switching is initiated and eventually completed. Thus, because current configurations lack an ATM-layer mechanism for immediately alerting the selector node of the occurrence of a failure, the selector node may have to wait a considerable amount of time before initiating the protection switching operation. This results in the destination node unnecessarily experiencing an interruption of the end-to-end connection.
Many deleterious effects can arise from delayed reaction to a failure condition. These include, but are not limited to, unnecessary loss of data and a failure condition being signalled to higher layers of the protocol. Consequently, there may result premature rerouting or termination of the connection.
Clearly, it would be desirable to provide a technique of initiating group protection switching more quickly than can be achieved using existing methods such as those disclosed in recommendation I.630.
According to the invention, the APS channel is assigned a higher priority than any of the traffic channels. This results in faster initiation of protection switching, as alarm messages are received on the APS channel before they are received on the traffic channels. Thus, protection switching can be started before the traffic channels have a chance to leave the protected domain carrying an alarm message.
Thus, a head-start is given to the protection switching mechanism, resulting in a shortening of the period during which the traffic channels carry alarm messages outside the protected domain. In some cases, protection switching can be completed even before the traffic channels have a chance to leave the protected domain carrying an alarm message.
Therefore, the invention may be summarized broadly as a method of protecting a group of traffic channels travelling through a protected domain. The protected domain includes a bridge node, a selector node and a plurality of other nodes arranged in a working path and in a protection path between the bridge node and the selector node. According to the invention, the method includes a node in the working path detecting failure of the traffic channels, after which the node transmits failure information to the selector node along the traffic channels and along an alarm channel in the working path.
Upon receipt of said failure information by the selector node along said alarm channel, the selector node initiates protection switching of the traffic channels. According to the invention, the alarm channel has a sufficiently high priority relative to the traffic channels to allow the selector node to initiate protection switching before failure information is received along any of the traffic channels.
Preferably, the traffic channels contain ATM virtual path connections or virtual channel connections and the alarm channel is an ATM virtual path connection or virtual channel connection.
The invention is applicable to a 1+1 protection scenario and to a 1:1 protection scenario. In a 1+1 protection scenario, the traffic channels travel through the protected domain along both the working path and the protection path prior to failure detection, and the selector node performs protection switching by selecting the traffic channels arriving along the protection path. In a 1+1 protection scenario, if the priority of the alarm channel is sufficiently high, the selector node can complete protection switching even before failure information is received on any of the traffic channels.
In a 1:1 protection scenario, the traffic channels travel through the protected domain along only the working path prior to failure detection. Protection switching is performed by the selector node transmitting a message to the bridge node along a second alarm channel in the protection path, the bridge node receiving the message and responding thereto by sending the traffic channels along the protection path and the selector node selecting the traffic channels arriving along the protection path.
The invention can also be summarized broadly as an intermediate node for connection in the working path of a protected domain. The intermediate node includes a unit for receiving traffic cells from a previous node and forwarding the received traffic cells to a next node, a unit for detecting a failure to receive traffic cells from the previous node and a unit for generating alarm cells and transmitting them to the next node in response to detecting a failure. The alarm cells contain failure information and have a higher priority than any of the traffic cells.
Moreover, the invention can be summarized broadly as an article of manufacture, including a computer usable medium having computer readable program code embodied therein for protecting of a group of traffic channels travelling through a protected domain. The computer readable program code in the article of manufacture includes computer readable program code for causing a computer to receive traffic cells from a previous node and forward the received traffic cells to a next node, computer readable program code for causing the computer to detect a failure to receive traffic cells from the previous node and computer readable program code for causing the computer to generate alarm cells and transmit said alarm cells to the next node in response to detecting a failure, wherein the alarm cells contain failure information and have a higher priority than any of the traffic cells.
The invention can also be summarized broadly as a selector node for connection at the end of a working path and at the end of a protection path in a protected domain. The selector includes a unit for receiving user information from a previous node in the working path along a plurality of working traffic channels and forwarding said user information to a node outside the protected domain. As well, the selector node includes a unit for receiving failure information from the previous node in the working path along a working alarm channel, where the working alarm channel has a higher priority than any of the working traffic channels. Finally, the selector node contains a unit for initiating protection switching immediately upon receipt of failure information along the working alarm channel.
Also, the invention may be summarized broadly as an article of manufacture which includes a computer usable medium having computer readable program code embodied therein. The code includes a portion for causing a computer to receive user information from a previous node in the working path along a plurality of working traffic channels and to forward the user information to a node outside the protected domain. As well, the code includes a portion for causing the computer to receive failure information from the previous node in the working path along a working alarm channel, where working alarm channel has a higher priority than any of the working traffic channels. Finally, the code contains a portion for causing the computer to initiate protection switching immediately upon receipt of failure information along the working alarm channel.
The invention may also be broadly summarized as a protected domain having a bridge node, a selector node and a plurality of intermediate nodes arranged in a working path and in a protection path, where the various nodes are adapted to perform any of the above methods.
Because the working alarm channel is given a higher priority than any of the traffic channels, protection switching can be initiated by the selector node prior to the receipt of alarm cells on any of the traffic channels. While there may be a period during which the traffic channels contain alarm cells, fast initiation of protection switching allows this period to be significantly reduced relative to that resulting from a conventional protection switching approach. As a result, the premature termination of the end-to-end traffic connection is prevented.